1. Field of the Invention
The present invention relates to image scanning apparatus. More particularly, the invention relates to conditioning the radiance of illumination in an object plane so that the irradiance at an image plane is uniform with field angle.
2. Description of the Prior Art
Image scanning is a process of converting an image recorded on a photosensitive sheet, such as photographic film, to an electronic image. Scanning, as such, is commonly employed as a first step in the transmission of an image from one storage medium to another, or in the enhancement or the analysis of the image prior to its transmission.
With a photographic film held in a given plane, an image scanner measures the optical density of the film by illuminating it with light of known intensity and measuring the amount of light transmitted through, or reflected from, the film. In doing so, the scanner effectively divides the image recorded on the film into discrete picture elements, or pixels, and assigns to each a number or value representing an average density for each pixel. Commonly, the pixels are arranged in rows and columns to form a two-dimensional grid with the density of each pixel corresponding to a relatively small portion of the overall image.
There are various types of image scanning systems known in the art. An image scanning system of the so-called serial type measures the density of only one film pixel at a time. A microdensitometer, for one, employs a very narrow stationary beam of light. An X-Y translation mechanism moves the film in two orthogonal directions relative to the stationary light beam so that a desired area of the film image is scanned.
A laser flying-spot scanner and a CRT flying-spot scanner are also of the serial type--one film pixel is measured at a time. With a flying-spot scanner, however, a movable beam of light scans the film, and lens apparatus, having an appropriate field of view, focuses light impinging on the film onto an image sensor.
A line scanner and an area scanner are two other systems known in the art. With a line scanner, an entire row (or column) of film pixels is illuminated simultaneously by projecting a line of light onto the film. A linear translation device moves the film in a direction orthogonal to the light line to scan the entire film image.
With an area scanner, all film pixels are scanned simultaneously by projecting a relatively broad beam of light onto the entire film image.
A line scanner and an area scanner, like a flying-spot scanner, employ lens apparatus having an appropriate field of view to focus light projected on the film image onto an appropriate image sensor, i.e. a linear sensor in the case of a line scanner and an area sensor (a 2-dimensional CCD, etc.) for an area scanner.
As is appreciated by those skilled in the image scanner art, accuracy in the measuring of film density values is an extremely important performance criterion. The irradiance in an image, however, varies as the field angle of an object or image increases. As disclosed in Applied Optics and Optical Engineering by William C. Wolfe, Volume VIII, page 142, irradiance in an image decreases in proportion to the fourth power of the cosine of the field angle, defined by an optical axis and a line connecting the center of the image and its corresponding object. Thus, an image scanner system that employs either a scanning light beam, a line of light, or a relatively broad light beam for illuminating an area (an image scanner other than a microdensitometer having a stationary narrow beam of light), suffers from a disadvantage in that irradiance at the image sensor varies with field angle.